The selection of grounding mode is the key part of DC micro grid design. In this paper, the grounding mode of DC micro grid is analyzed based on the example of Yanqing 31.5 MW Badaling photovoltaic power station. Firstly, this paper introduces the operation mode, topology structure and several grounding methods of DC micro grid. Secondly, different grounding modes are analyzed in theory from the steady-state characteristics, fault characteristics and personal safety aspects. Finally, the simulation model of the DC micro grid is established in PSCAD/EMTDC, and the theoretical analysis is verified by the electromagnetic transient simulation. The results show that, compared with other grounding methods, the power supply side is more suitable for low voltage DC micro grid system through the IT grounding mode with large resistance grounding.